Many electronic circuits contain active elements such as bipolar transistors or unipolar Field Effect Transistors (FETs). These electronic circuits may contain one active element or several elements configured in various arrangements to provide specific functions, such as an RF power amplifier, power switch, etc. Generally, the class of microwave amplifiers employing bipolar and field effect transistors are referred to as two-port devices. Such devices act as valves. They control a large amount of power being delivered to an output load by a DC power supply with a small input signal. The input and output ports present a positive resistance to the circuit.
To serve their designed function, these active devices must be properly biased. Biasing is achieved usually from a single DC supply of a given polarity depending upon whether the device is an n or p type device. However, more than one DC supply can be used as well. In any event, the input port of the device, as well as the output port, must be separately biased at different voltage levels. Thus one biases the input port of the device, such as the base electrode of a transistor or the gate electrode of an FET and the output port, such as the collector or emitter electrode of a transistor or the drain or source electrode of an FET.
Many conventional test systems are known which are used to test and characterize active devices. These test systems provide DC biasing techniques which activate the device under test. Biasing voltages are applied from DC supplies, essentially in a continuous wave or CW mode. The biases are continuously and quiescently applied during the entire test. Moreover, the RF signal source used as the test signal at the input port of the device under test has also been applied in a CW or quiescent mode.
A disadvantage of these test systems has been that careful attention had to be given not to exceed the power limitations of the device under test. Since the DC bias and the RF signal were applied in a CW manner, power dissipation was also critical. The device under test had to be mounted on a good heat sink medium to ensure that the heat created was dissipated away from the device. Devices producing relatively high levels of RF power or requiring high levels of DC bias power, which were mounted on a medium having poor heat conductivity, could not be tested. This was especially true with hybrid devices that were still mounted on a wafer.
In order to reduce heat dissipation, other conventional systems are known which apply an RF signal in a pulse modulated manner. One such system is the Hewlett Packard 85110A pulsed S-parameter test set, which measures the scattering parameters of an active device by use of a pulse modulated RF signal. There is no capability, however, for controlling the DC power dissipation of the active device under test.
It is, however, the object of the present invention to overcome the disadvantages and limitations of prior active circuit test systems by providing a new and improved test system.
Another object of the present invention is to provide an active circuit test system which allows control of power dissipation while providing accurate RF and bias data for the circuit under test.